1. Field of the Invention
This invention relates to Metalorganic Chemical Vapor Deposition (MOCVD) reactors and more particularly to susceptors used in MOCVD reactors.
2. Description of the Related Art
Fabrication of gallium nitride (GaN) based semiconductor devices in MOCVD reactors is generally described in DenBaars and Keller, Semiconductors and Semimetals, Vol. 50, Academic Press Inc., 1997, p. 11-35. MOCVD is a nonequilibrium growth technique that relies on vapor transport of the precursers and subsequent reactions of group III alkyls and group V hydrides in a heated zone. Growth/source gasses and dopants are supplied to the reactor and are deposited as epitaxial layers on a substrate or wafer. One or more wafers usually rest on a structure of graphite called a susceptor that is heated by a heating element such as a radio frequency (RF) coil, resistance heated, or radiantly heated by a strip heater. The heated susceptor then heats the wafers, which allows for the source gasses to form epitaxial layers on the wafers.
FIG. 1 shows a conventional susceptor 10 that is used in MOCVD reactors such as those provided by Thomas Swan Scientific Equipment Limited. It has a hollowed cylindrical shape and is mounted over the reactor's heating element at the bottom of the reactor, below a source gas inlet. It has a circular base plate 12 and cylindrical sleeve 13, with the circular plate 12 having a series of circular depressions 14 equally spaced around the susceptor's longitudinal axis. Each of the depressions 14 can hold a semiconductor wafer during growth. When the susceptor 10 is heated by the heating element the semiconductor wafers are also heated and when source gases enter the MOCVD reactor, they combine and deposit on the heated semiconductor wafer as epitaxial layers. The susceptor 10 can typically spin at speeds in the range of 1,000 to 2,000 rpm, which results in more uniform epitaxial layers on the wafers.
Conventional susceptors 10 are usually formed from a monolithic structure of graphite or coated graphite that absorbs heat from the heater element and conducts it to the wafers in contact with the susceptor 10. The entire susceptor 10 is heated uniformly to achieve consistent growth conditions across the entire surface of the wafers. However, during fabrication of the epitaxial layers, materials not only deposit on the heated wafer, but can also deposit on the heated susceptor 10. For example, during growth of Group III Nitride based devices, significant amounts of GaN, InGaN, AlInGaN, and similar compounds can deposit on the susceptor surfaces. The result is a buildup of reaction deposits on the susceptor that can adversely impact subsequent fabrication steps. The deposits can act as impurities during subsequent growth of the epitaxial layers and they can also result in poor interface transitions between subsequent layers of different compositions. For example, if a layer using an indium source gas is grown on the wafers, indium can be deposited on the susceptor 10. If the next layer to be grown does not include indium, indium from the susceptor surfaces can be included in the transition between layers and in the next layer. These impurities can cause poor device performance and can prevent consistent reproduction of semiconductor devices on the wafer. This deposition of materials on the susceptor surfaces also results in more reactants being consumed than is necessary for the formation of devices on the wafers.
Another disadvantage of conventional susceptors is that because the heating element heats the entire susceptor (not just the areas under or around the wafers) large amounts of heat are required. Conventional susceptors have a relatively large surface area in comparison to the wafers and most of the energy is wasted by not heating the wafers. This taxes the heater, contributing to early heater failures.
Another disadvantage of conventional susceptors is that they are difficult to manufacture. They are machined from a large section of graphite and if any part of the susceptor is damaged the entire structure can be unusable. The fabrication of the depressions can be extremely difficult because they are offset from the structure's longitudinal axis and as a result, they cannot be machined using a simple lathe, but must involve more complex processes. In some susceptors it may also be desirable to shape the surface of the depressions to compensate for variations in temperature. For the same reasons that it is difficult to machine the depressions, it also difficult to shape the surface of the depressions.
Various “inverted” type systems have been developed to grow semiconductor devices, wherein the susceptor is not mounted at the bottom of the reactor. An inverted type metal organic vapor phase epitaxy (MOVPE) system for the growth of Group III-V compound semiconductor materials is described in Aria et al., Highly Uniform Growth on a Low-Pressure MOPVE Multiple Wafer System, Journal of Crystal Growth 170, Pgs. 88-91 (1997). The wafers are held in a susceptor and placed facedown (inverted) in the growth chamber, with the flow gasses flowing under the growth surfaces. Gasses are injected into the chamber from one of the sidewalls of the chamber, through a triple flow channel, and the gas exhaust is on the opposite sidewall.
The fluid flow and mass transport for “chimney” chemical vapor deposition (CVD) reactors is discussed in Holstein, Modeling of Chimney CVD Reactors, Journal of Crystal Growth 125, Pgs. 311-319 (1992). A chimney reactor has wafers held on heated susceptors (usually two) that are vertically mounted on the interior side walls of the reactor.
Growth of GaAs based semiconductor devices in an MOCVD reactor is also discussed in Lee et al. MOCVD in Inverted Stagnation Point Flow, Journal of Crystal Growth, Pgs 120-127 (1886). The reactor is based on inverted stagnation point flow geometry where the reactants flow up towards wafers clamped to an inverted heated susceptor.
Each of these inverted type systems use conventional susceptors that are usually formed from a monolithic structure of graphite or coated graphite. They also suffer from the disadvantages described above.